A power generation system may used to generate power for either a stationary or a vehicular application. For example, an electric power generation system may used to provide electric power for a building or to provide power for propelling a vehicle and/or operating systems of a vehicle that require electric energy. In particular, a power generation system may be used to provide electric energy for what is sometimes referred to as a “hybrid vehicle” or “hybrid-electric vehicle,” which may include a combustion engine operably associated with an electric generator. The combustion engine generates mechanical energy and the electric generator converts at least a portion of the mechanical energy into electric energy. The electric energy may be used to operate one or more electric motors and/or other electrically-operated vehicle systems. For example, the one or more electric motors may be used to provide torque to driving members (e.g., wheels or ground engaging tracks) to propel the vehicle either independently or in combination with torque provided by the combustion engine. For a vehicle such as a work machine, for example, a wheel loader or a track-type tractor, the one or more electric motors may be used to propel the vehicle and the mechanical energy produced by the combustion engine and/or electric energy produced by the generator may be used to operate work implements via hydraulic actuators and/or hydraulic motors.
Regardless of whether the power generation system is used in a stationary or a vehicular application, it may often be desirable for the power generation system to be able to quickly and efficiently respond to sudden increases in load on the system. For example, in a power generation system used to provide electric energy for a building, if a sudden large increase in demand for electric energy occurs due, for example, to activation of one or more appliances requiring a substantial amount of electricity, a sudden large load will be placed on the power generation system. In vehicle applications, for example, in a work machine application, if the work machine is traveling across the ground and lifts a heavy load, such as a bucket filled with dirt and rock, a sudden large load will be placed on the power generation system due to the mechanical energy needed to lift the load. Such sudden large loads may cause the engine speed of the combustion engine to drop to an undesirably low speed (sometimes referred to as an “under-speed condition”), which may result in inefficient and/or undesirable operation of the combustion engine.
In order to counteract such large sudden loads on the electric power generation system, it may be desirable for the combustion engine to provide a responsive increase in torque to the power generation system. The rate at which the sudden electric and/or mechanical load may occur, however, may be essentially instantaneous relative to the ability of the combustion engine to respond to the sudden load. In particular, in order for the combustion engine to provide the desired responsive increase in torque, an increase in the amount of fuel and/or air supplied to the combustion engine must be increased. For example, more fuel may be delivered to the combustion engine to increase torque. In combination with the reduced engine speed that accompanies the sudden load, however, the resulting combustion may exhibit unacceptably high exhaust emission levels due to an overly rich air-to-fuel ratio (AFR). On the other hand, the intake pressure (e.g., the inlet manifold pressure) of the combustion engine may be increased in order to deliver more air to the combustion engine. If, however, the amount of fuel delivered to the engine is not also increased, an acceptable AFR will not likely be achieved, and there will not be a sufficient increase in torque to counteract the sudden load on the power generation system.
One technique for increasing the intake pressure of a combustion engine is the use of an exhaust gas-driven turbocharger. Such a turbocharger uses energy contained in the gases exhausted following combustion in a combustion engine to spin a turbine operably coupled to a compressor that, in turn, compresses air delivered to the intake system of the combustion engine. In this fashion, the intake pressure may be increased and more air may be delivered to the combustion engine, thereby increasing its output torque. Due to its exhaust gas-driven nature, however, a turbocharger may take an undesirably long time to respond to the need for increased output torque in response to a sudden change in load. In particular, since the turbocharger's compressor relies on the exhaust gas for driving its turbine, at low engine speeds that may occur as a result of a sudden load increase, the turbocharger's ability to provide a responsive increase in intake pressure may be undesirably slow.
One method of increasing the speed of response of a turbocharger is described in U.S. Pat. No. 4,901,530 (the '530 patent) issued to Kawamura on Feb. 20, 1990. The '530 patent describes a method for controlling a turbocharger with a rotary electric machine that detects the rotational speed of a motor vehicle's engine and an amount of accelerator pedal depression to monitor running conditions of the motor vehicle. A required boost pressure based on the running conditions is determined and if a difference between the required boost pressure and a present boost pressure is greater than a prescribed value, the device determines that the motor vehicle is running under conditions that require quick acceleration. The device then drives the rotary electric machine coupled to the rotatable shaft of the turbocharger to increase the speed of rotation of the turbocharger for a quick buildup of the boost pressure.
Although the device of the '530 patent may speed the build-up of boost pressure of the turbocharger, the determination that the motor vehicle is running under conditions that require quick acceleration are based on the detected rotational speed of a motor vehicle's engine and an amount of accelerator pedal depression. For power generation systems that experience sudden loads, however, detecting a motor's rotational speed and an accelerator pedal depression may not provide an effective determination basis and/or a quick enough response for increasing the rotation of the turbocharger's compressor in order to provide an increase in a combustion engine's torque output.
The disclosed electric turbocharger system is directed to overcoming one or more of the problems set forth above.